The present invention relates to wireless telecommunication systems, and more particularly to methods and apparatuses that enable multiple radio systems to operate in similar radio spectrums and/or located geographically near to each other.
When a few decades ago, spectrum regulations were changed to allow commercial radio applications in unlicensed bands, interest was marginal. But this interest has changed radically in the last few years. After the worldwide success of mobile telephony in licensed bands, capacity limitations and huge license fees have spurred the interest of radio applications in the unlicensed band. In the past few years, communications systems such as those operating in accordance with the Wireless Local Area Network (WLAN) IEEE 802.11 standards and the Bluetooth® standards have been increasingly deployed in the 2.4 GHz band. Moreover, new communications systems are being worked on, such as the Wireless Personal Area Network (WPAN) activity under IEEE 802.15.
Radio spectrum, even unlicensed, is limited. Despite this, ubiquitous communications using several different standards is foreseen in the near future. Coexistence is not trivial as different standards follow different protocols. Moreover, regulations, initially intended to provide fair sharing, are constantly changing to allow for higher data rates, yet moving away from robustness requirements. The use of an unlicensed band poses the challenge of coexistence. In the design phase of a new communication system that has to operate in the unlicensed band, the developer has to design units that will be expected to share the band with:
Incumbent non-communications: Power unintentionally radiated by equipment, for example microwave ovens, will be a source of disturbance.
Incumbent communications: Intended radiation by other communication systems like for example WLAN, Bluetooth®, or Radio Frequency-Identification (RF-ID) will also be experienced as disturbance when no coordination is applied.
Future systems: Systems that do not exist yet but which will be built in the future can cause severe disturbances. The only known factors are the restrictions imposed upon these systems by the regulations. However, as discussed before, regulations are changing over time, making predictions rather unreliable.
Coexistence can be handled in a number of different ways, as will now be discussed.
Interference mitigation by applying direct-sequence spreading or forward-error-correction coding can be useful, but is usually insufficient due to the near-far problem. That is, in ad-hoc scenarios in particular, a jamming transmitter can come very close to a receiver. The power levels received can thus be sufficiently strong to bring the front-end of the receiver into saturation, which causes clipping. As a result of the clipping (which imposes non-linear effects) the effective gain decreases (desensitization) and intermodulation products arise.
Avoidance is another method of mitigating interference. Avoidance in time can be applied by listening-before-talk or Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) as applied in IEEE 802.11 and other standards. However, this renders suboptimal solutions because the collision measurements render absolute power levels whereas the Packet Error Rate (PER) depends on the Carrier-to-Interference (C/I) ratio.
Avoidance in frequency is provided by frequency agile techniques such as Dynamic Frequency Selection (DFS). In this method, the system measures where in the frequency band other transmitters are active, and subsequently avoids these frequency segments. This is fine when potential jammers broadcast their presence continuously, for example on a control channel. However, measuring on bursty data channels results in unreliable measurements. Hopping provides better mitigation methods based on frequency avoidance. Because of the large isolation between the intended signal and the jammer when the hopper and jammer do not coincide, rather good robustness can be obtained. However, frequency hopping only works when the jammers are narrowband; likewise, time hopping only works when jammers have a low duty cycle. Incumbent systems in the unlicensed bands usually are bandwidth restricted but are rarely duty cycle restricted, posing a problem for time hopping systems like Ultra-Wideband (UWB) Impulse Radio.
Arranging for the different systems to use different frequencies is another coexistence technique. However, when the different transceivers are located in the very same device or are otherwise very close to one another, practical problems relating to filtering out strong interference result in the use of different frequency bands being insufficient to avoid interference between the different systems unless those different frequency bands are sufficiently separated from one another.
More particularly, when the different systems are sufficiently separated in frequency, coexistence is typically ensured by means of filtering. In this way the systems can be treated independent of one another, as if the other systems were not at all present. In such cases, the performance of each system will be unaffected of operation of the other systems. The cost associated with this approach is the possibility of very hard requirements on the necessary filters. In addition, a filter also causes attenuation loss of the desired signal, known as the insertion loss. This results in degraded sensitivity for systems that employ such hard filtering.
When the different systems operate very close to one another in frequency, filtering is generally not a feasible solution, due to the very hard requirement on the attenuation required to ensure that the victim system will not be severely degraded.
Thus, often the only feasible way of coexistence is the use of time division, in which the systems are coordinated in time so that no two systems are active at the same time. One issue related to coexistence by means of time division is that some kind of collaboration between the systems usually is needed. For instance, if it is known by one system that another system is receiving, the former might delay its transmission not to interfere with the latter. Alternatively, the latter system might choose not to use the information received in case it knows the former is transmitting, and instead rely on that the information can be obtained anyway through powerful coding and time interleaving or possibly by retransmission mechanisms.
Coordination between the systems is, for example, the typical approach used when Bluetooth® and WLAN technology are co-located in the same device. A method known as Packet Traffic Arbitration (PTA) is used, and this results in one of the two standards being given priority over the other one.
A problem with using coexistence solutions based on time division is that the systems that are given low priority might not work well. For instance, it might not be possible to guarantee the desired Quality of Service (QoS) of one system if there is another active system having a higher priority. Another problem is the poor utilization of the available spectrum. If only one system is used at a time, parts of the spectrum will always be unused for data transmission, and effectively used only as a guard band.
Today, the ISM band at 2.4-2.485 GHz is used both by Bluetooth® technology and by WLAN. Both of these incompatible technologies can be found in mobile phones, and the percentage of phones that will have both Bluetooth® and WLAN technology built into them will increase in the future. The bands used for the cellular standards, like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA) are today located several hundred MHz away from the ISM band, and ensuring co-existence between for instance Bluetooth® technology and the cellular standards has been easily achieved by means of filtering. However, with the introduction of technology built in accordance with the Worldwide Interoperability for Microwave Access (WiMAX) standard, which might be used in the 2.3 GHz band, filtering might not be sufficient to ensure co-existence. Also, with the International Mobile Telecommunications-2000 (IMT-2000) extension band located at 2.5-2.69 GHz, filtering alone will not suffice to ensure coexistence with standards using the 2.4 GHz ISM band.
As these various communication devices become smaller, the number of transceivers in different devices like mobile phones, personal digital assistants (PDAs), laptop computers, and the like is increasing. This means that co-existence between different systems is an issue that can be expected to become even more pronounced in the future.
Therefore, it is desirable to have methods and apparatuses that enable various radio communication systems to coexist with one another in an efficient way.